1. Field of the Invention
This invention relates to an alumina ceramic containing a small, strengthening addition of zirconia; the ceramic is one which can be produced by a method that lends itself to mass production.
2. The Prior Art
The subject of alumina-zirconia ceramics produced both by hot pressing and by sintering techniques has received a great deal of attention during recent years. A journal article, Cer. Eng. and Sci. Proc., Vol 1,7-8(B) 1980, is considered to be typical of the prior art relating to such ceramics made by hot pressing. The article, D. Lewis III and P.F. Becher, "Thermal Shock Behavior in Al.sub.2 O.sub.3 -based composites", reports test data indicating that aluminazirconia composites which were studied are highly resistant to thermal shock. The data relate to alumina ceramics and to ceramics composed of alumina and up to 30 percent by volume of ZrO.sub.2 produced by hot pressing at 1500-1600 and 35 MPa (about 5075 pounds per square inch). The data presented indicate the alumina-zirconia ceramics to have outstanding thermal shock properties. Another journal article, J.Am.Cer.Soc., 61, No. 12, pp. 85, 86, and U.S. Pat. No. 4,218,253, are illustrative of the prior art relating to the production of such ceramics by sintering. The patent discloses (Example 1) the production of an aluminazirconia ceramic from aluminum oxide powder (average particle size 5 microns) and monoclinic zirconium oxide powder (average particle size 1 micron). The process involves wet mixing the two powders, drying and granulating the mixture, isostatically pressing a shape from the granules, and sintering the shape at 1600 for one hour. The journal article discloses a similar process, including sintering at 1500 and 1600, but is silent as to particle size, disclosing only that "composites with a very fine and homogeneous dispersion" were achieved "by a wet-chemical method, starting from a zirconium sulfate-aluminum sulfate solution." It will be appreciated that hot pressing alumina-zirconia ceramics at 1500-1600 and 35 MPa is a costly procedure, that even sintering at 1600 is costly, and that alumina produced by a wet chemical method from a zirconium sulfate-aluminum sulfate solution is a costly starting material. Accordingly, as might be expected, the ceramics produced by the methods of the subject references, and all other alumina-zirconia ceramics that have been suggested by the known prior art, are costly and have found only limited commercial use, for example as tool bits.
Japanese patent application No. 53-126008, Nov. 2, 1978, discloses the production of what has been translated as an "aluminum ceramic" having improved thermal shock resistance by pressing discs from ceramic batches and firing to 1065.degree.. Batches containing 94 to 96 parts by weight of Al.sub.2 O.sub.3, 1 to 2 parts by weight of CaO, 1 part by weight of MgO, 2 to 3 parts by weight of SiO.sub.2, up to 2 parts by weight of Cr.sub.2 O.sub.3 and up to 10 parts by weight of ZrO.sub.2 are disclosed.
The present inventor discovered a ceramic composed of a mixture of particulate Al.sub.2 O.sub.3, particulate ZrO.sub.2 and glass bonding the Al.sub.2 O.sub.3 and the ZrO.sub.2 into a dense, gas impervious structure, and also found that all or any part of the particulate ZrO.sub.2 in such ceramics can be replaced by particulate HfO.sub.2 or by a solid solution of HfO.sub.2 and ZrO.sub.2, in particulate form, as well as that Y.sub.2 O.sub.3 can advantageously be present to stabilize at least a part of the ZrO.sub.2, the HfO.sub.2 or the ZrO.sub.2 -HfO.sub.2 solid solution in a cubic crystalline structure. The glass constitutes from 1 to 15 percent of such ceramics, while particulate Al.sub.2 O.sub.3 constitutes from 75 to 85 percent, based upon the weight of the ZrO.sub.2, HfO.sub.2, Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3. "Manning", U.S. Pat. No. 4,552,852, is directed to this discovery.